1. Field of the Invention
The present invention relates to ionomer resins that provide for superior properties for a variety of articles of manufacture, including golf balls. The present invention also relates to methods of manufacture of these resins.
2. Description of Related Art
Ionomers, at their most general definition, are ion-containing polymers. Ionomeric resins generally available are ionomeric copolymers of an olefin and a metal salt of a unsaturated carboxylic acid, or ionomeric terpolymers having a co-monomer within their structures. These resins vary in resiliency, flexural modulus, and hardness. Examples of these resins include those marketed under the trademark SURLYN manufactured by E.I. DuPont de Nemours & Company of Wilmington, Del., and IOTEK manufactured by ExxonMobil Corporation of Irving, Tex. These materials are chosen for use in a variety of applications, because they provide good properties with respect to cost, weight, and durability. These marketed ionomers are described in greater detail below.
These copolymeric and terpolymeric ionomers may be described as copolymer E/Y or terpolymer E/X/Y, where E represents ethylene, X represents a softening co-monomer such as acrylate or methacrylate, and Y represents acrylic or methacrylic acid. In particular, the acid moiety of Y is neutralized to form an ionomer by a cation such as lithium, sodium, potassium, magnesium, calcium, barium, lead, tin, zinc or aluminum. Also, a combination of such cations is used for the neutralization. Copolymeric ionomers are obtained by neutralizing at least a portion of the carboxylic groups in a copolymer of an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, with a metal ion. Examples of suitable α-olefins include ethylene, propylene, 1-butene, and 1-hexene. Examples of suitable unsaturated carboxylic acids include acrylic, methacrylic, ethacrylic, α-chloroacrylic, crotonic, maleic, fumaric, and itaconic acid. Copolymeric ionomers include ionomers having varied acid contents and degrees of acid neutralization, neutralized by monovalent, or multivalent cations discussed above.
Terpolymeric ionomers are obtained by neutralizing at least a portion of the carboxylic groups in a terpolymer of an α-olefin, and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylate having 2 to 22 carbon atoms with metal ion. Examples of suitable α-olefins include ethylene, propylene, 1-butene, and 1-hexene. Examples of suitable unsaturated carboxylic acids include acrylic, methacrylic, ethacrylic, α-chloroacrylic, crotonic, maleic, fumaric, and itaconic acid. Terpolymeric ionomers include ionomers having varied acid contents and degrees of acid neutralization, neutralized by monovalent or multi cations discussed above.
As stated above, ionomers are used in a variety of applications. In particular, ionomers are used in the manufacture of sports equipment, such as golf balls. Golf balls generally include a core and at least one cover layer surrounding the core. Balls can be classified as two-piece, multi-layer, or wound balls. Two-piece balls include a spherical inner core and an outer cover layer. Multi-layer balls include a core, a cover layer and one or more intermediate (or mantle) layers. The cover and intermediate layers themselves may include multiple layers. Wound balls include a core, a rubber thread wound under tension around the core to a desired diameter, and a cover layer, typically of balata material. Material characteristics of the compositions used in sports equipment, including golf ball layers are important in determining the durability and performance of the equipment. For example, with respect to golf balls, the composition of a golf ball cover layer is important in determining the ball's durability, scuff resistance, speed, shear resistance, spin rate, feel, and “click” (the sound made when a golf club head strikes the ball). Various materials having different physical properties are used to make cover layers to create a ball having the most desirable performance possible.
Ionomers are particularly useful in compositions for making golf ball covers, because they can provide a good mix of durability, hardness, and feel. Often, ionomers are blended in cover compositions. An ionomer having a high flexural modulus often is combined in a cover composition with another ionomer having a low flexural modulus. The resulting intermediate-modulus blend will provide for acceptable hardness, spin, and durability of the resulting cover. These ionomer blends also can be readily used with various fillers, fibers, colorants, and processing aids commonly used in polymer blends. However, even with blending of ionomers to improve properties, their use is not completely satisfactory. Improving one characteristic can lead to worsening another. For example, blending an ionomer having a high flexural modulus with an ionomer having a low flexural modulus can lead to reduced resilience and durability compared to use of the high-modulus ionomer alone. In general, it is difficult to make a material for an ionomeric golf ball cover layer to provide low hardness, good feel, high speed, high resilience, and good shear durability.
To improve upon the properties of conventional ionomers, Spalding Sports Worldwide, Inc. (“Spalding”) and E.I. dupont de Nemours and Co. (“DuPont”) each have patented conventional ionomers modified by metal fatty acids. In an exemplary ionomer of this type, the addition of sodium stearate increases the COR of the resulting ionomer when used in for example, golf balls, without increasing the hardness of the ionomer. DuPont also has patented conventional ionomers incorporating a bimodal molecular weight distribution, which can be used to adjust the properties of the ionomers. Examples of references disclosing modified ionomers include U.S. Pat. No. 6,100,321 and U.S. Published Patent Application No. 2003/0158312, both to Chen. DuPont and other companies also have recently been increasing the acid neutralization in these ionomers to be almost 100%, also to adjust the properties of the ionomers.
Despite the variety of modified and high-neutralization ionomers currently available, no ionomer exists that provides for ideal properties of COR, compression, durability, and ease of processing. In view of the above, it is apparent that a need exists for improved polymers for use in a variety of applications, such as sports equipment. The present invention fulfills this need and provides further related advantages.